basic_dict = {
	'1': ['1', '3'],
	'2': ['1', '4'],
	'3': ['2', '3'],
	'4': ['2', '4']
}

find_sku_dict = {}
for (key, value) in basic_dict.items():
    new_key =  '_'.join(value)
    find_sku_dict[new_key] = key
print(find_sku_dict)



find_sku_dict = {
	'1_3': '1',
	'1_4': '2',
	'2_3': '3',
	'2_4': '4',
}